LABOUR is demanding a full independent investigation into an “apparent breach of civil service neutrality” after it was suggested Whitehall officials questioned whether Jeremy Corbyn was “too frail” to become Prime Minister.

Unite leader Len McCluskey, a close ally of the 70-year-old Labour leader, insisted he was as “fit as a fiddle,” and denounced the “fake news…lies…and distortion” of the Murdoch Press, which ran the story.

On Saturday, Mr Corbyn branded the Times report "a farrago of nonsense" and said the briefing to a newspaper by senior UK Government officials against a politician "should be very concerning" to people.

Jon Trickett, the Shadow Minister for the Cabinet Office, has written to Sir Mark Sedwill, the Cabinet Secretary and Head of the Civil Service, to demand an investigation, independent of the Cabinet Office, into what he called the “apparent breach of civil service neutrality”.

In the letter, Mr Trickett describes the matter as a "totally unwarranted and indeed unconstitutional political intervention with disturbing implications for our democratic system" and formally requests a meeting with Sir Mark to discuss the matter.

On Saturday, one unnamed official was quoted as saying: “There is a real worry that the Labour leader isn't up to the job physically or mentally but is being propped up by those around him. There's growing concern that he's too frail and is losing his memory. He's not in charge of his own party.”

There was even a claim Mr Corbyn had suffered a mini-stroke, strenuously denied by Labour.

One Labour source was quoted as saying: “Jeremy is just a puppet, he can barely hold his head up. He is being manipulated and controlled.”

The claims were angrily dismissed by Mr McCluskey, who said the reporters responsible for the report "ought to be ashamed of themselves".

He went on: "It was fake news, it was lies, it was distortion. Jeremy Corbyn is fit as a fiddle, he is one of the strongest people I have ever met; people 20 years younger struggle to keep up with him...”

The union leader added: "I don't believe them at all; it's the Murdoch press. Of course, they are going to attack and try to undermine Jeremy."

Meanwhile, John McDonnell, the Shadow Chancellor, admitted that shifting Labour's stance to full support for Remain and a second referendum was taking longer than expected.

Mr McDonnell said the discussion was about the "attitude" Labour should take and that Mr Corbyn had given trade unions more time to decide.

“He had a meeting on the Monday before the last Shadow Cabinet and I thought the next day we were going to move forward.

"The unions asked for a bit more time and I don't mind that; that's the type of leader Jeremy is, he is a consensus builder," Mr McDonnell told Sky’s Sophy Ridge on Sunday.

Mr McCluskey insisted there should be no "panic" to switch Labour's position on Brexit as he blamed a "well-funded Remain lobby" for creating a "toxic situation".